The present invention relates to waves generated by marine vessels, more particularly to methodologies for controlling or preventing potentially hazardous wake wash conditions such as are caused by marine vessels traveling at high speeds in shallow waters.
Increasing numbers of high speed ferries are navigating coastal and inland waterways. In addition, both commercial and military entities are developing other types of vessels, particularly large displacement vessels, for operation in shallow waters such as coastal and littoral waters. The wake wash caused by vessels traveling in shallow waters represents both an environmental threat and a safety threat. The potential for damage and injury caused by wake wash is real. Wake wash created by vessels navigating shallow waters can result in shoreline erosion, structural damage, human injury or loss of life. Wake wash can wreak havoc on beaches, sand banks and sea walls and on property in such locales. Wake wash can unexpectedly arise on a calm day, thus posing a risk to people in the vicinity who are caught unawares.
The United States and other countries have responded to wake wash risks in various ways, including speed restrictions, ferry scheduling, breaking wave height standards, risk assessments for prospective operator licensees, legal actions, and public protests. The most prevalent approaches to limiting wake wash are based entirely on speed limits for specific near-shore regions. Such approaches to wake wash mitigation tend to miss some areas in need of speed restrictions. Furthermore, the speed limits imposed may be excessively or unnecessarily restrictive.
Wake wash waves created by a vessel are a function of vessel speed and water depth. It is generally accepted principle—and testing has shown—that the worst, most damaging wake wash waves are generated at or around a Froude depth number of 1.0. The Froude depth number equals the quantity vessel speed divided by the square root of the product water depth times gravitational constant. That is, the Froude depth number Fnd is calculated using the formulaFnd=V×(d×g)−0.5 where V is the vessel speed, d is the water depth and g is the gravitational constant. The term “critical speed” has been used to describe a speed resulting in a Froude depth number equal to one. The term “subcritical speed” has been used to describe a speed resulting in a Froude depth number less than one. The term “supercritical speed” has been used to describe a speed resulting in a Froude depth number greater than one.
Since the Froude depth number is a function of both vessel speed and water depth, wake wash alleviation measures that only take vessel speed into account represent—in principle and often in practice—an incomplete solution to the wake wash problem. The most common mitigative actions currently taken by regulatory bodies involve the mere imposition of speed limits. Speed limits fail to address the real issue of the physical relationship of wake wash with both vessel speed and water depth. Speed limits are not useful for every location; they can be either less restrictive or more restrictive than necessary, because the water depth is not taken into account.
Feldtmann U.S. Pat. No. 6,171,021, incorporated herein by reference, proposes physical alteration of the ocean floor. Feldtmann discloses a wake wash reduction approach involving the formation along a water bed of a “transition area” in which the natural water depth has been artificially altered through human engineering. Feldtmann asserts that, during the vessel's transition from subcritical speed to supercritical speed or from supercritical speed to subcritical speed, a vessel's speed can be adjusted while traveling in the transition area so as to avoid critical speed. Although Feldtmann discloses a solution having some conceptual validity insofar as both vessel speed and water depth are considered, his solution is not entirely realistic and does not lend itself to widespread practice.
The following references, informative on the subject of wake wash, are incorporated herein by reference: Trevor Whittaker, Bjorn Elsaber, “Coping with the wash: The nature of wash waves produced by fast ferries,” Ingenia, quarterly of the Royal Academy of Engineering, Issue 11, Feb. 2002, pages 40–44; Trevor Whittaker, A. Bell, M. Shaw, K. Patterson, “An investigation of fast ferry wash in confined waters,” paper no. 13, presented at the International Conference of Hydrodynamics of High Speed Craft, Royal Institution of Naval Architects (RINA), London, 24–25 Nov. 1999 (13 pages); I. W. Dand, T. A. Dingham-Peren, L. King, “Hydrodynamic aspects of a fast catamaran operating in shallow water,” paper no. 11, presented at the International Conference of Hydrodynamics of High Speed Craft, Royal Institution of Naval Architects (RINA), London, 24–25 Nov. 1999 (17 pages); Alan Blume, “High-speed vessel wake wash,” slide presentation presented at Ship Effects Workshop, U.S. Army Engineer Research and Development Center (ERDC), Gulfport, Miss., 29–30 Oct. 2002 (15 pages); “A physical study of fast ferry wash characteristics in shallow water,” Final Report, Research Project 457, 2000, The Maritime and Coastguard Agency (United Kingdom), Nov. 2001 (115 pages); R Allen and R. Clements, “Ship wash impact management (SWIM),” presented at the Sixth International Conference on Fast Sea Transportation (FAST 2001), Royal Institution of Naval Architects (RINA), London, Sep. 2001, pages 91–96; R. Doyle, T. Whittaker, B. Elsaber, “A Study of Fast Ferry Wash in Shallow Water,” presented at the Sixth International Conference on Fast Sea Transportation (FAST 2001), Royal Institution of Naval Architects (RINA), London, Sep. 2001, pages 107–119.